The present invention relates to an airfoil, engine and engine nacelle combination, where the aerodynamic contours are such as to properly accommodate the engine and approach minimum drag.
It has long been known that aerodynamic interference drag can be alleviated by contouring an object so that its surface lies as nearly as possible along streamlines generated by adjacent aerodynamic objects. With regard to a nacelle mounted adjacent to a wing, an idealized configuration would be such that the nacelle is "hidden" aerodynamically from the nearby wing by shaping the nacelle so that it lies entirely along streamlines generated by the wing. In this case, the nacelle would be "invisible" to the wing and thus no interference would occur. This is a well understood concept of aerodynamics and has been disclosed in various publications (e.g. (a) Report No. NASA CR-2905, dated Sept. 1977, entitled "Wind Tunnel and Analytical Investigation of Over-the-wing Propulsion/Air Frame Interference For A Short Haul Aircraft at Mach Numbers from 0.6 to 0.78.)" (b) AIAA Paper Number 73-9, entitled "Recent Advances in Aerodynamics for Transport Aircraft", by L. T. Goodmanson and L. B. Gratzer.
In order to meet this requirement, the nacelle would lie along a stream tube of the air passing adjacent the wing, and the volume of the nacelle would be determined entirely by that available within the stream tube. If the nacelle were mounted to a pylon, then the pylon would have to be infinitely thin, so as to lie entirely on a stream sheet. Or, if the pylon had thickness, it would have to extend from infinity upstream to infinity down stream, being between two stream sheets of the air flowing around the wing.
With regard to a nacelle containing an engine this ideal nacelle and pylon are not practical for a number of reasons. Among the more important reasons are the following. First, the nacelle cross-sectional area at its main control portion is substantially larger than at its inlet or outlet area, because of the turbine machinery, the nacelle structure and accessory gear boxes. Also, the inlet stream tube tends to expand as it nears the inlet. Another consideration is that the engine exhaust tends to change shape after it leaves the exit nozzle and generally has a different area than the stream tube that it is replacing. Where there is a pylon, the pylon must have thickness, which in turn requires a leading and trailing edge which are by necessity near the wing.
Thus in actual practice a nacelle and pylon must depart in some manner from the ideal geometry which would produce no interference with the adjacent airstream. It is an object of this invention to provide a wing, engine, and engine nacelle combination arranged in a manner that each performs its intended function and with the overall combination being arranged in such a manner as to minimize drag.
In a search of the patent literature, there was disclosed a U.S. Pat. No. 3,199,813, Roper, which relates to contouring a pod mounted to an outboard end of a swept wing. That portion of the pod which is either directly above or directly below the wing is contoured so as to follow the two-dimensional flow patterns of the airstream flowing directly over or directly under the wing. While the Roper patent does represent an advance in the state of the art at that time, to the best knowledge of the applicants, the teaching in Roper is not adequate to provide a full understanding of the principles necessary to arrive at the present invention.
The other patents noted in the patentability search are representative of various aerodynamic shapes. A review of these patents indicate that to the best knowledge of the applicants herein these are not particularly relevant to the present invention. However, these are disclosed herein to ensure that the applicants are fully complying with their duty of disclosing to the Patent Office all prior art of possible relevance.
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